1. Field of the Invention
The present invention relates to a printing system that prints images on both surfaces of a web.
2. Related Art
There has been proposed a printing system that prints images on both surfaces of a web, which is in a continuous belt shape. The printing system includes a pair of print devices arranged in a row, wherein a web is formed with an image on its front surface by a first print device, discharged from the first print device, turned upside down by a reversing device, fed into a second print device, and then formed with an image on its rear surface.
FIG. 1 shows components of an electrophotographic print device that can be used in such a printing system. In this device, a photosensitive drum 101 is formed with an electrostatic latent image on its surface at a position EP. Then, toner is selectively supplied onto the surface of the photosensitive drum 101, so that a visible toner image corresponding to the electrostatic latent image is formed on photosensitive drum 101. When the toner image comes into contact with a web W at a transfer point TP, the toner image is transferred onto the web W. Afterwards, the toner image is thermally fused onto the web.
This thermal fusion shrinks the web, so that the web has a length shorter than its original length. Accordingly, when an electrophotographic print device is used as the first print device, then there is a danger of positional deviation between the front surface image and the rear surface image on the web W.
However, when n-number of sprockets are formed in each page of the web, then it is possible to avoid such a positional deviation by transporting the web while counting the number of the sprockets in the second print device. That is, transporting the web by n-sprocket-worth of distance means transporting the web by a single-page worth of distance regardless of whether or not the web has shrunk. Because a location of the sprocket in each page head of the web never changes, it is possible to transfer a single-page worth of toner image from the photosensitive drum 101 onto a corresponding page of the web by transporting the web by a distance equivalent to n-number of sprockets. Subsequent pages can also be formed with corresponding single-page worth of images in the same manner.
In other words, if each page head of the web W meets a corresponding page-head position of the photosensitive drum 101 at the transfer point TP, the rear surface image is formed in a positional alignment with a corresponding front surface image, and there is no danger that positional deviation accumulates to greatly deviate the positional relationship between downstream-side front and rear surface images.
Here, when a printing process is started, a web transport speed is accelerated to a predetermined speed. Since it is necessary to reach the predetermined speed before the page head of the first page of the web reaches the transfer point TP, the web is positioned, after a previous printing operation has completed, such that the page head of a first page for a subsequent printing operation locates at a predetermined waiting position WP. The waiting position WP is downstream from the transfer point TP with respect to the web transport direction by a distance xcex1, which is required to reach the predetermined speed.
However, when the last-page images are completely transferred onto the web W at the transfer point TP, the page head of the first page for the subsequent printing process has already passed the waiting position WP. Therefore, after the printing has completed, the web W is transported back to the waiting position in the following manner.
When an electrostatic latent image for the last page is completely formed on the photosensitive drum 101, a CPF-OFF signal is generated. When a predetermined time T1 elapses after the CPF-OFF signal was generated, then a PF position clear signal is generated. Here, the time duration T1 is a time between when the CPF-OFF signal is generated and when the page head of the subsequent first page reaches the waiting position WP, and is expressed by the formula:
T=(L1xe2x88x92xcex1)/VP xe2x80x83xe2x80x83(1) 
wherein L1 is a moving distance of the photosensitive drum 101 from the position EP to the transfer point TP;
xcex1 is a distance from the waiting position WP to the transfer point TP; and
VP is a process speed of the second print device, which equals to the rotational speed of the photosensitive drum 101.
Upon reception of the PF position clear signal, a measuring unit starts measuring a web transport distance. After the toner image for the last page is completely transferred onto the web W, the web transport is stopped. Then, the web W is transported in a reverse direction by the amount of the web transport distance measured by the measuring unit. In this manner, the web is transported back to the waiting position WP.
There is also provided a printing system of a type that uses a web formed with no sprockets. In this type of printing system, positioning marks are used instead of the sprockets for achieving the positional alignment between the front-surface images and the rear-surface images. More specifically, the first print device prints positioning marks on a predetermined position in each page in addition to the images. A detection unit of the second print device detects the positioning marks and outputs output signals accordingly. Then, the second print device controls the web transport speed such that the output timings of the output signals have the constant phase with respect to CPF-N signals which are generated periodically. The control of the web transport speed is necessary since the web W has a different length between when the front surface printing and when the rear surface printing as mentioned above.
As described above, when the web with no sprockets is used, the web transport speed is controlled to change during the printing in the above described manner. Therefore, if the PT position clear signal is output when the predetermined time T1 elapses after the CPF-OFF signal was generated, then the web W may not be positioned at the waiting position WP.
Such a problem does not occur in the conventional printing system of a type that uses a web W with sprockets, since the web transport speed can be maintained constant in this case.
In the view of foregoing, it is an object of the present invention to overcome the above problems, and also to provide a method for accurately positioning a web with no sprockets at a predetermined waiting position in a print device.
In order to achieve the above and other objects, according to the present invention, there is provided a printing system including a first printing means for printing images on a first surface of a web, a second printing means for printing images on a second surface of the web, and a control means for controlling both the first and second printing means. The second printing means includes a photosensitive member, an irradiating unit, a developing means, a calculating means, a transport means, and a measuring means. The irradiating unit irradiates a laser light onto the photosensitive member for forming latent images thereon. The control means generates a reference signal when the irradiating unit completes irradiating a laser light for a last page image. The developing means develops the latent images into toner images. The calculating means calculates a time duration required for the web to reach a predetermined waiting position after the reference signal was generated. The transport means stops transporting the web in a forward direction after a last-page toner image is completely transferred from the photosensitive member onto the web. The measuring means measures a web transport distance between when the time duration elapses from when the reference signal was generated and when the transport means stops transporting the web in the forward direction. The transport means transports the web in a reverse direction by the web transport distance measured by the measuring means so as to transport the web back to the web waiting position. The calculating means calculates the time duration based on a web transport speed at the time of when the reference signal was generated.
There is also provided a control method for controlling a second printing means of a printing system that includes a first printing means for printing images on a first surface of a web and the second printing means for printing images on a second surface of the web, the second printing means including a photosensitive member, an irradiating unit that irradiates a laser light onto the photosensitive member, and a web transport means for transporting the web. The control method includes the steps of a) generating a reference signal at a time of when the irradiating unit completes irradiating a laser light for a last page image, b) calculating a time duration required for the web to reach a predetermined waiting position after the reference signal was generated, c) controlling the web transport means to stop transporting the web in a forward direction after a last-page image is completely transferred from the photosensitive member onto the web, d) measuring a web transport distance by which the web has been transported between when the time duration has elapsed from when the reference signal was generated and when the web transport was stopped in the step c), and e) controlling the web transport means to transport the web back to the waiting position, by transporting the web in a reverse direction by the web transport distance measured in the step d). The time duration is calculated in the step b) based on a web transport speed at the time of when the reference signal was generated.